Die bonding to a board

ABSTRACT

A method of bonding a plurality of die having first and second metal layers on a die surface to a board, comprising placing a first die onto a board comprising one of a ceramic or substrate board or metal lead frame having a solderable surface and placing the first die and the board into a reflow oven. The method includes reflowing at a first reflow temperature for a first period until the first metal board layer and at least one of the first and second metal die layers of the first die form an alloy to adhere the first die to the board. The alloy has a melting temperature higher than the first reflow temperature. Accordingly, additional die may be added at a later time and reflowed to attach to the board without causing the bonding of the first die to the board to fail.

This application claims priority to U.S. Provisional Application No.62/194,204, filed on Jul. 18, 2015, entitled “DIE BONDING TO A BOARD,”assigned attorney docket no. ONS01778P, invented by Michael J. Seddon etal.

CROSS-REFERENCE TO RELATED APPLICATIONS

Related subject matter is found in a copending patent applicationentitled “FLIP CHIP BONDING ALLOYS”, U.S. patent application Ser. No.______, filed ______, by Michael J. Seddon et al. and assigned to theassignee hereof.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to circuit manufacturing, andmore particularly to device bonding techniques.

BACKGROUND

Multichip modules are increasingly being used to control operations ofmachines and systems. For a variety of manufacturing considerations,however, the various devices are not always installed into the multichipmodules at the same time. Wire bond applications are often used thoughnot always appropriate for certain applications. For example, a wirebond solution may not be appropriate in situations where footprintrequirements necessitate efficient integrated circuit (IC) real estateusage.

An alternative approach is to use solder alloys and pastes. Using solderin either form is beneficial for bonding one die to a board, such as aceramic or substrate board or metal lead frame, in certaincircumstances. Typically, the solder alloy or paste will flow when thedevice and board are placed in a reflow oven or furnace and will thencool to bond the die to the board and, if desired, to create anelectrical connection between the device and board. For the case of asingle die, this approach is beneficial in that the die may be removedand replaced if faulty with a subsequent reflow in a reflow oven orfurnace.

One issue with using a solder alloy or paste, however, is that thesolder will reflow and the bond between the original die and the boardwill fail when the board and a new die are subsequently placed in thereflow oven to bond the new die to the board. When the bond fails, theoriginal die may even slide off of the board or metal lead frame. Oneapproach to solve this problem has been to use different metal alloysfor the original or first die so that the metal alloys bonding the diehave a higher melting temperature than what is needed for re-flowing thesolder alloys of the second or new die. Two metals that have been usedwith this approach include lead and gold. Lead, however, is known tocause health issues and is not always desirable. Gold, on the otherhand, is expensive and drives up product cost. Accordingly, solder isoften used despite its limitations.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood, and its numerousfeatures and advantages made apparent to those skilled in the art byreferencing the accompanying drawings according to various embodimentsin which:

FIG. 1 is a side view of a circuit board and a first die prior tobonding the first die to the circuit board.

FIG. 2 is a side view of a circuit board and a bonded first die and anunbonded second die.

FIG. 3 is a side view of a circuit board and a bonded first die andsecond die.

FIG. 4 is a side view of a circuit board and a bonded first die and asecond die.

FIG. 5 is a temperature-time graph that illustrates a reflow thermalprofile for a first die.

FIG. 6 is a temperature-time graph that illustrates a reflow thermalprofile for a first die and a second reflow for a second die.

FIG. 7 is a flow chart illustrating a method for bonding a die.

FIG. 8 is a flow chart illustrating a method for bonding a die.

The use of the same reference symbols in different drawings indicatessimilar or identical items. Unless otherwise noted, the word “coupled”and its associated verb forms include both direct connection andindirect electrical connection by means known in the art, and unlessotherwise noted any description of direct connection implies alternateembodiments using suitable forms of indirect electrical connection aswell.

DETAILED DESCRIPTION

FIG. 1 is a side view of a circuit board and a first die prior tobonding the first die to the circuit board according to an embodiment.FIG. 1 illustrates a system that includes a die 10 having metal layers12 and 14 and a board 16 with metal layer 18. Board 16 may beconstructed in any known manner and may comprise, for example, a ceramicor organic substrate board. Alternatively, a metal lead frame may beused in place of a board. Hereinafter, whenever any reference to aboard, a ceramic board, a substrate board or a metal lead frame is used,it should be understood that any one of the other supporting structuralelements (board, substrate board, ceramic board or metal lead frame)could be used alternatively.

Metal layer 18 of board 16 comprises at least one of a solderable metallayer such tin, silver or copper. Metal layer 18 may also comprise aplurality of metal or metal alloy layers, for example, copper, nickel,tin or silver or an alloy formed of two or more metals. The metal layerson the die comprise a combination of silver, tin and nickel in oneembodiment. The die may include three or more layers. While FIG. 1 showstwo layers 12 and 14 on die 10 and one layer 18 on board 16, theembodiments are not limited and the die and board may include additionallayers. Moreover, any one layer may be an alloy and is not required tobe a single element or metal. For example, layer 14 also labeled as m2may be an alloy in one embodiment.

As is suggested in FIG. 1, die 10 is to be attached to board 16 (morespecifically, to layer 18 of board 16). In the prior art, a solder pasteor die attach is often used to bond the die to the board. Here, however,the metals or metal alloys of the plurality of metal layers on the dieand at least one layer on the board are selected so that they melt andform an alloy during reflow having a subsequent melting temperature thatis higher than the temperature used during reflow of the original metallayers to create the alloy. In the described embodiment, metals arechosen so that, even if a solder or solder paste is used, the metalswill melt and mix with the solder to create an alloy with a meltingtemperature that is higher than what is required to reflow solder. To“reflow” a board and one or more die, the board is often placed within areflow furnace or oven at a specified temperature profile over aspecified period. Other technologies for reflowing the metals may alsobe used in alternative processes. Any reference herein to a furnace oroven should be understood to include alternative technologies formelting or reflowing the metal layers of the die and board.

FIG. 2 is a side view of a circuit board and a bonded first die and anunbonded second die according to an embodiment. More specifically, die10 and board 16 are shown after a reflow wherein metals of at least oneof layers 12 and 14 of die 10 melted and mixed with layer 18 of board 16to create alloy layer 20 to bond die 10 to board 16. Alloy layer 20,being formed by the specific combination of metals m2 and m3 (andpotentially m1) has a higher melting (reflow) temperature than any oneof metals m1, m2 and m3 of layers 12, 14 and 18, respectively. In theexample of FIG. 2, all of metal m2 has mixed with metal m3 to createalloy m4 of layer 20.

Here, in FIG. 2, die 22 having layers 24 and 26 is shown disposedagainst layer 18 of board 16 but is not yet attached or bonded to board16. Accordingly, the board and die 10 and die 22 are reflowed in thereflow oven or furnace to cause metals m3 and m6 (and potentially m5) tomelt and reflow to create the bond between die 22 and layer 18 of board16. If the same reflow temperature is used as before, metals m6 and m3(and possibly m5 of layer 24) will melt to create a new alloy withoutcompletely melting alloy m4 of layer 20. Depending on operation, alloym4 may partially or slightly melt, but will not melt so completely thatthe bond between the die and the board is compromised to the point thatthe die may be removed or may even accidentally slide off of the board.

FIG. 3 is a side view of a circuit board and a bonded first die andsecond die according to an embodiment. In the example of FIG. 3, somemetal m2 of layer 14 remains while some has mixed with metal m3 of layer18 to create alloy m4 of layer 20. This is in contrast to FIG. 2 whereall of metal m2 of layer 14 melted and mixed with metal m3 of boardlayer 18. As before, alloy m4 was created during a first reflow.Additionally, some of metal m6 of layer 26 remains while some of metalm6 has mixed with metal m3 of layer 18 to create alloy m7 of layer 28.Alloy m7 was created during a second reflow at the first reflowtemperature without completely reflowing (melting) metal m4 of layer 20during the second reflow to bond or attach die 22 to board 16. If board16 was to be reflowed a third time to add another die, either alloy m4or m7 will not completely melt as long as the reflow temperature is thesame as before or at least lower than the melting temperature of metalsm4 and m7 of layers 20 and 28, respectively.

FIG. 4 is a side view of a circuit board and a bonded first die and asecond die according to an embodiment. Referring to FIG. 4, a die 30 isshown bonded to board 36 after a reflow. Prior to the reflow, die 30included at least three metal layers similar to die 46 that includeslayers 48, 50 and 52. In the illustrated embodiment of FIG. 4, die 30includes original layers 32 and 34 and new alloy layers 42 and 44. Board36 includes layers 38 and 40 that may each be a pure metal or an alloy.During the reflow, the original layer of die 30 included metal m3 (anpotentially a portion of layer 34 comprising m2) that mixed with atleast one of metals m4 and m5 of layers 38 and 40 of board 36 to createlayers 42 and 44 that comprise at least one alloy.

In one embodiment of the invention, a process includes reflowing a dieuntil at least two metals melt and mix to create an alloy. As anadditional process step, the reflow may be extended in time to a secondperiod to cause at least a third metal, if not a fourth metal, of eithera die layer or a board layer to mix to create additional alloys. FIG. 4illustrates the results of this additional process step wherein twometal alloys shown as layers 42 and 44 were created through re-flowingan extended or additional time to partially or wholly combine metals m2,m3, m4 and m5. Furthermore, the process may optionally include extendingthe reflow time to create additional alloys from the various metallayers of the die and board that comprise metals m1-m5. As before, whendie 46 displaced onto board 36 and re-flowed, alloys m6 and m7 of dielayers 42 and 44 will not completely melt and reflow. Accordingly, board36 will maintain a bond with die 30.

It should be understood that distinct layers are shown here to representalloys with differing combinations of metal. In actual practice,however, the ratios of metal may gradually change depending on originalmetal layer thicknesses and a total period of the reflow process tocreate the alloys. Accordingly, it should be understood that the term“intermetallic alloy” or “intermetallics” may be used in place of alloymore appropriately depending on the results of the reflow process.Reflow duration and relative layer thickness and construction affect howmuch the various metal layers melt and how well they mix to create auniform distribution of metals within the resulting alloy. For example,if the metal compositions are different in different areas, the term“intermetallic alloy” may be more appropriate than the term “alloy”because “alloy” tends to refer to a homogenous or uniform distributionof metal content. References herein to alloy are intended to includeintermetallic alloys.

FIG. 5 is a temperature-time graph that illustrates a reflow thermalprofile for a first die according to one embodiment. Referring now toFIG. 5, a reflow thermal profile graph is shown that identifies fivetime periods p1-p5 that represent the various thermal stages used in aprocess according to one embodiment. As may be seen, period p1represents a heating ramp to preheat the reflow oven or furnace to apreheat temperature. Period p2 represents the preheat period that isused in a typical reflow process. Period p2 may be used to burn offimpurities or flux and/or to gradually increase temperature to avoiddamaging the board or die. Period p3 represents a heating ramp to areflow temperature. Period p4 represents the duration that a reflowtemperature is maintained. Typically, in prior art applications, areflow temperature is not maintained for any notable duration given thetraditional solder elements that are used to bond leads and devices toeach other or to a printed circuit board. Here, however the reflowtemperature is maintained at least until a time t1 is reached.

Time t0 is the time at which the reflow temperature is reached whiletime t1 is an amount of time that is required to form an alloy andcreate a bond. Time t1 may coincident with time t0. Time t1 is afunction of what metals are being melted and the relative thickness ofthe metal layers. Furthermore, as described in relation to previousfigures, a plurality of metal layers may be used on at least one of thedie or the board. If the reflow temperature is maintained beyond time t1to time t2, additional alloys may be formed from the additional metallayers disposed either on the die or on the board because maintainingthe reflow temperature causes additional melting. Accordingly, shortlyafter time t2, period 5 begins representing a cool down period. Asdescribed before, the alloys that are formed by the reflow temperatureat times t1 and t2 have subsequent melting temperatures that are higherthan the reflow temperature of period p4. The process represented byFIG. 5 is a reflow process for a first die.

FIG. 6 is a temperature-time graph that illustrates a reflow thermalprofile for a first die and a second reflow for a second die accordingto one embodiment. Essentially, the reflow thermal profile illustratedin FIG. 6 is the equivalent of that in FIG. 5 except that the profile isrepeated. The left-hand portion of the reflow thermal profile of FIG. 6represents the reflow process for a first die while the right-handportion represents the reflow process for a second die to which thefirst die is subjected. The descriptions for the various periods made inrelation to FIG. 5 are the same here and won't be repeated. The reflowprocess for the second die is performed with the first die being alreadybonded to the board. Because of the characteristics and higher meltingtemperatures for the alloys that are formed during the first reflow,however, the first die and its metal layers do not reflow (completely orfully melt to the point that the bond between the board and die iscompromised) while the metal layers for the second die are re-flowing inthe reflow oven or furnace during the second reflow cycle illustrated inFIG. 6.

FIG. 7 is a flowchart illustrating a method for bonding a die accordingto one embodiment. The method of FIG. 7 commences with placing a firstdie on a solderable board with metal layer (102). The solderable boardmay comprise any type of known board including ceramic boards andsubstrate boards or metal lead frames that are used for securing orconnecting electronic devices. In one embodiment, the metal layer is asolderable metal layer such as copper. The die, in the disclosedembodiment, includes two metal layers formed on one surface while theboard includes at least one solderable metal surface. The die is placedon the board so that the outer metal layers of board and first die arein contact with each other. Thereafter, the first die and the board areplaced in a reflow furnace or oven (104). The board and die kept in theheated reflow oven to reflow the first die and the solderable boardmetal layer according to a first reflow temperature profile to form afirst alloy (106).

The temperature profile may include a temperature ramp to a specifiedtemperature with an immediate cool down following or a ramp to atemperature or temperature range that is maintained for a specifiedperiod or duration sufficient to allow the metal layers to reflow tocreate at least one alloy. Reference herein to temperature profilesinclude any combination of temperature and time that is used to melt thedie and board metal layers to create the desired alloys and/orintermetallic alloys. The method optionally includes continuing thereflow according to a second temperature profile (e.g. at the firstreflow temperature profile for a second period) to continue to reflowthe first die and one or more of the metal layers of the die and theboard to form a second alloy or intermetallic alloy (108).

After a first reflow process is concluded and any formed alloys havecooled and hardened, the method includes placing a second die on thesolderable board (110) board with the first and second die in the reflowfurnace or oven (112) to essentially repeat the reflow process.Thereafter, second die and board metal layers are reflowed according toa third reflow temperature profile without completely re-flowing anyalloy metals of the first die and board that were created during thefirst reflow process. The second die and board metal layers arere-flowed to form a third alloy for the second die and board (114).Finally, the method optionally includes continuing to reflow the metallayers of the second die and board according to a fourth temperatureprofile to form a fourth alloy for the second die (116). It should beunderstood that the first, second, third and fourth temperature profilesmay be similar or may be varied in thickness or composition. Similarly,the first, second, third and fourth alloys and/or intermetallics thatare created are based on the temperature profiles and metal layercompositions and may therefore be similar or different.

FIG. 8 is a flowchart illustrating a method for bonding a die accordingto one embodiment. Initially, a first die is placed against a boardwithout a solderable paste. As described before, the die has at leasttwo metal layers and the board has at least one metal layer that issolderable in one embodiment (120). Thereafter, the die and the boardare heated to a first temperature to burn off flux and/or impurities(122). In some processes, a flux is added to external metal layers toavoid unwanted oxidation that may interfere with reflow processes. Inone embodiment, the die and board are placed into a reflow oven orfurnace to eventually reflow the metal layers to create an alloy orinter-metallic layer (a layer differing metal compositions throughoutthe metal). Thereafter, the board and die are heated to a secondtemperature to reflow the board metal layer with at least one metallayer of the die to form a first alloy (124). In one embodiment, theambient temperature is reduced from the second temperature as soon asthe second temperature is reached. For some metal combinations, once theambient temperature reaches a specified reflow temperature, the metalsare sufficiently melted and a cool down process may begin immediatelyunless other metal layers are also to be melted to form the desiredalloy.

Optionally, the board and die are kept at the second temperature for aspecified period. The method continues with continuing to maintain heatto continue to reflow the board metal with first and second die metallayers to form a second alloy (126). This step includes maintaining aspecified temperature range (e.g., the second temperature reached instep 124) for a specified period. In the described embodiment, a solderor solder paste was not used. Alternatively, for the method of FIG. 8, asolder or solder paste may be used in addition to the metal layers ofthe board and die when the die is placed against the board so long as ametal melts and mixes with the solder or solder paste to create an alloywith a higher melting temperature than the melting temperature of thesolder.

After the first and second alloys have cooled enough to bond the die tothe board, the method includes placing a second die against the boardwithout solderable paste (128). Thereafter, the method optionallyincludes heating the second die and the board with the attached firstdie to the first temperature to burn off flux and/or impurities (130).Thereafter, the reflow oven or furnace temperature is increased to heatthe board according to a fourth temperature to reflow the board metallayer and the second die first metal layer to form a third alloy (132).As described in relation to step 124, the fourth temperature may be thesame as the second temperature or it may be modified according to designrequirements. Finally the method concludes with maintaining the heat atthe fourth temperature or within a temperature range approximately equalto the fourth temperature to reflow the board metal layer with thesecond die first and second die metal layers to form a fourth alloy(134). As before, this step may comprise merely maintaining a specifiedtemperature or temperature range for a specified period. It should beunderstood that the first and second die might have substantiallysimilar metal layers thereby resulting in substantially similar alloysfrom the reflow processes. In other words, the third and fourth alloysmay be very similar to the first and second alloys, respectively if themetals and temperature profiles or processes are similar.

The above-disclosed subject matter is to be considered illustrative, andnot restrictive, and the appended claims are intended to cover all suchmodifications, enhancements, and other embodiments that fall within thetrue scope of the claims

Thus, to the maximum extent allowed by law, the scope of the presentinvention is to be determined by the broadest permissible interpretationof the following claims and their equivalents, and shall not berestricted or limited by the foregoing detailed description.

What is claimed is:
 1. A method of bonding a plurality of die having aplurality of metal layers on a die surface to a board or metal leadframe, comprising: placing a first die onto a solderable surface of theboard or the metal lead frame, the board comprising one of a ceramicboard or substrate board, or a metal lead frame wherein a top metal dielayer is disposed against the solderable surface; first reflowing atleast one of first and second metal die layers of the first die at afirst reflow temperature for a first period to form a first alloy tocreate a bond between the first die and the board or metal lead frame;and wherein the first alloy has a melting temperature that is higherthan the first reflow temperature.
 2. The method of claim 1, the firstreflowing further including reflowing at the first reflow temperaturefor a second period wherein the solderable surface and a second metallayer of the die to form a second alloy.
 3. The method of claim 2wherein the solderable surface comprises copper and the first and secondmetal die layers comprise silver and tin, respectively.
 4. The method ofclaim 2 wherein the first includes at least a third metal layer.
 5. Themethod of claim 4 wherein at least one layer comprises nickel and atleast one layer comprises silver.
 6. The method of claim 4 furtherincluding reflowing at the first reflow temperature for a third periodlong enough to cause the solderable surface and at least one of thesecond and third metal layers on the die to form a third alloy.
 7. Themethod of claim 4 wherein the first reflow is for a period that causesan alloy to form comprising nickel, silver and tin.
 8. The method ofclaim 1 wherein the board comprises a ceramic board with copper andsilver layers and wherein the die comprises nickel, silver and tinlayers.
 9. The method of claim 1 wherein the first alloy meltingtemperature is in the range of 220 degrees C. to 260 degrees C.
 10. Themethod of claim 1 wherein the melting temperature is greater than 260degrees C.
 11. The method of claim 1 wherein the second meltingtemperature is at least 10 degrees C. higher than the first meltingtemperature.
 12. The method of claim 1 further including: placing asecond die onto the solderable surface wherein a top metal die layer ofthe second die is disposed against the solderable surface; secondreflowing at the first melting temperature for the first period untilthe solderable surface and at least one metal die layers of the seconddie form either the first or a second alloy to adhere the second die tothe board; and wherein the first alloy formed between the first die andthe board or metal lead frame does not completely melt and the first dieremains adhered to the board or metal lead frame during the secondreflowing.
 13. A method of bonding a die to a board, comprising: placinga first die having a first solderable surface against a board or metallead frame without using a solder paste, the board or metal lead framehaving a second solderable surface, the board further comprising one ofa ceramic or substrate board; wherein the first solderable surface ofthe first die comprises a first plurality of metal layers and wherein anouter layer of the first solderable surface comprises a silver and tinalloy having a silver composition that is less than 7 percent by weightand further wherein an inner layer next to the outer layer is either ametal layer or metal alloy layer that comprises one of titanium, nickelor silver; and reflowing the first die at a first reflow temperature toform an additional alloy having a melting temperature that is higherthan the first reflow temperature wherein the additional alloy bonds thefirst die to the board.
 14. The method of claim 13 wherein thesolderable layer of the board comprises at least one of copper, silverand tin layers.
 15. The method of claim 13 further includingsubsequently placing a second die having a second solderable die surfaceonto the solderable layer of the board or metal lead frame and reflowingat the first reflow temperature without reflowing the additional alloyof the first die to create a second additional alloy having a meltingtemperature that is higher than the first reflow temperature.
 16. Themethod of claim 15 further including at least one of: initiallyreflowing the first die for a second period to create at least a thirdadditional alloy; and subsequently reflowing the second die for a secondperiod to create a fourth additional alloy.
 17. An apparatus,comprising: a first solderable layer on a surface of a ceramic orsubstrate board or a metal lead frame; first circuitry comprising: afirst solderable die surface on a first die comprising a first pluralityof metal layers wherein an outer layer comprises a silver and tin firstalloy having a silver composition that is less than 7 percent by weightand wherein an inner layer next to the outer layer comprises one oftitanium, nickel or silver; and a second alloy having a subsequentmelting temperature that is higher than a first reflow temperaturerequired to adhere the first die to the first solderable layer of theboard, wherein the second alloy bonds the first alloy to the firstsolderable layer of the surface of the board to hold the die to theboard; and second circuitry comprising: a second solderable die surfaceon a second die comprising a third alloy; and wherein the first andsecond circuitry are configured to control operations of the apparatus.18. The apparatus of claim 17 wherein the second alloy has a meltingtemperature that will not completely melt during subsequent reflowprocess at the first reflow temperature used to add the secondcircuitry.
 19. The apparatus of claim 17 wherein the first solderablelayer of the ceramic or substrate board or metal lead frame comprises atleast one of copper, silver and tin.
 20. The apparatus of claim 18wherein the subsequent melting temperature of an alloy that is createdduring a previous reflow is approximately at least 15 degrees C. higherthan a reflow temperature during the previous reflow that reflowed themetals and created the alloy.